1. Field of the Invention
The present invention relates generally to control devices and control methods of internal combustion engines, and particularly to a technology of controlling an internal combustion engine having a changing mechanism that changes opening/closing timing of at least one of an intake valve and an exhaust valve.
2. Description of the Background Art
A VVT (Variable Valve Timing) mechanism that is operated by a hydraulic pressure to change opening/closing timing (phases) of an intake valve and/or an exhaust valve has been known. The VVT mechanism changes the opening/closing timing to achieve an optimum amount of air supplied into a cylinder as well as an optimum amount of EGR (Exhaust Gas Recirculation) according to an operation state of an internal combustion engine. A hydraulic fluid of the VVT mechanism has characteristics that change significantly according to a temperature of the hydraulic fluid. Therefore, it is desired to give consideration to the temperature of the hydraulic fluid for accurately controlling the VVT mechanism.
Japanese Patent Laying-Open No. 2004-68809 has disclosed a control method for CVVT (Continuously Variable Valve Timing) of an automobile engine, and particularly a method of estimating a fluid temperature from a start state of the engine and controlling the CVVT according to an estimated value. The control method disclosed in Japanese Patent Laying-Open No. 2004-68809 includes a step of calculating an estimated fluid temperature value of CVVT control oil or fluid (i.e., oil or fluid for CVVT control) based on a cooling water temperature in an initial stage after the start as well as an elapsed time after the start, a step of storing the estimated fluid temperature thus calculated in a memory, and a step of driving the CVVT based on the estimated fluid temperature value thus stored.
However, even when the VVT mechanism is controlled in consideration of the temperature of the hydraulic fluid, as is done in the control-method disclosed in Japanese Patent Laying-Open No. 2004-68809, a sufficient hydraulic pressure may not be supplied to the VVT mechanism because the hydraulic fluid has a low viscosity, e.g., at a high temperature. In this case, the VVT mechanism may malfunction, and the opening/closing timing of the valve may not be controlled accurately.